Orthomyxoviridae are a family of RNA viruses which, so far as is known, infect mainly vertebrates (Thogotovirus in ticks, Isavirus in the sea louse). It includes those viruses which cause influenza.

There are three genera of influenza virus, identified by antigenic differences in their nucleoprotein and matrix protein:Influenzavirus A are the cause of all flu pandemics and are known to infect humans, other mammals and birds (see also avian influenza),Influenzavirus B are known to infect humans and seals,Influenzavirus C are known to infect humans and pigs.

There are three types of influenza virus: Influenzavirus A, Influenzavirus B or Influenzavirus C. Influenza A and C infect multiple species, while influenza B almost exclusively infects humans.[2]

The type A viruses are the most virulent human pathogens among the three influenza types and causes the most severe disease. The Influenza A virus can be subdivided into different serotypes based on the antibody response to these viruses.[2] The serotypes that have been confirmed in humans, ordered by the number of known human pandemic deaths, are:H1N1 caused "Spanish Flu".H2N2 caused "Asian Flu".H3N2 caused "Hong Kong Flu".H5N1 is a pandemic threat in 2006-7 flu season.H7N7 has unusual zoonotic potential.[3]H1N2 is endemic in humans and pigs.H9N2, H7N2, H7N3, H10N7.

Influenza B virus is almost exclusively a human pathogen, and is less common than influenza A. The only other animal known to be susceptible to influenza B infection is the seal.[4] This type of influenza mutates at a rate 2-3 times lower than type A[5] and consequently is less genetically diverse, with only one influenza B serotype.[5] As a result of this lack of antigenic diversity, a degree of immunity to influenza B is usually acquired at an early age. However, influenza B mutates enough that lasting immunity is not possible.[6] This reduced rate of antigenic change, combined with its limited host range (inhibiting cross species antigenic shift), ensures that pandemics of influenza B do not occur.[7]

The influenza C virus infects humans and pigs, and can cause severe illness and local epidemics.[8] However, influenza C is less common than the other types and usually seems to cause mild disease in childrenhttp://encyclopedia.thefreedictionary.com/orthomyxovirus